As-deposited coating and coatings heat treated at 200 and 600 °C have been characterized; the one heat treated at 400 °C, which did not differ from the one heat treated at 200 °C, has not been further examined.
The qualitative behavior of the 200 °C heat treated coating, in terms of friction coefficient evolution and wear scar morphology, does not differ significantly from the behavior of the assprayed coating, but, quantitatively, the wear rate is higher both against alumina and against 100Cr6 (Table 1). The wear rate for both coatings is higher against 100Cr6 than against alumina.
Table 1 Pin-on-disk test results
In both cases, the friction coefficient follows a peculiar evolution (Fig. 1): in the first stage of the test, the friction coefficient soon reaches a very high value. After a certain sliding distance, shorter in the case of alumina counterpart than for steel counterpart, a great amount of oxide debris is formed, and simultaneously the friction coefficient markedly decreases to a minimum value, retained for a very short distance (a few meters). Then, friction coefficient increases again. In the case of the alumina pin, friction then seems to attain a stable value. In the case of the steel pin, it again shows a peak and a decrease before reaching a seemingly stable value (this stable value is indicated in Table 1). A tentative explanation for this phenomenon is discussed later in this paper.
Fig. 1 Friction coefficient for the as-sprayed coating tested against 100Cr6 steel and sintered alumina pins over a 250 m sliding distance
Significant adhesive wear probably occurs for as-sprayed and 200 °C heat treated coatings tested against 100Cr6 steel; indeed, evidence of this wear mechanism can be found, for instance, on the as-sprayed coating wear scar (Fig. 2a, area in rectangle). As described, adhesive wear occurs because surface asperities of the two bodies plasticize when coming into contact, since the actual contact pressure they have to bear is significantly higher than the nominal one; the real contact area, in fact, is definitely smaller than the nominal one due to the rough, nonideal profile of any real surface. Plastic deformation of contacting asperities results in the formation of junctions; that is, the asperities are locally “welded” together. To keep relative motion between the two bodies, junctions must be broken up: the fracture may occur either at the exact junction point, or inside one of the two bodies. In the latter case, the fracture generally occurs inside the surface having lower hardness (since hardness is connected to local yield strength) and results in direct wear loss on this surface, with material transfer to the counterbody. In the former case, no direct wear loss occurs, but asperities undergoing repeated plastic deformation are subject to a very severe fatigue process, which soon causes the nucleation and growth of a surface crack, resulting in the detachment of very small, plateletlike wear debris. This debris can remain in the contact area, be expelled, or stick to one of the surfaces, thus contributing to the formation of the transfer film. In this case, evidence of small asperities being torn away from the coating surface and transferred to the pin surface is clearly present in Fig. 2a (see area in the box) and 2b (transferred material on the pin appears as small protrusions on its surface). Energy dispersive spectroscopy (EDS) analysis confirms that Co, Mo, Cr, and Si are the main constituents of this transferred material. Some pin wear loss also occurs; however, steel pin wear rates are one order of magnitude lower than the coating ones, and no significant material transfer occurs from pin to coating. Adhesive wear obviously causes much friction, since a high tangential force must be applied to break junctions apart; this can be the reason for the high friction coefficient in the first stage of the test. It must also be noticed that high friction results in high flash temperatures in the contacting asperities: much heat is generated on the small real contact area due to friction. Thus, the local hardness of contacting asperities decreases and adhesion progressively increases, further favoring high friction. Besides, as the film of transferred material on the pin surface grows, a larger part of the contact involves surfaces with the same chemical composition: higher chemical affinity increases friction.
Fig. 2 SEM micrograph of wear scar on (a) as-sprayed coating and (b) 100Cr6 steel pin. The rectangle indicates evidence of adhesive wear, the arrow indicates dark oxides.
本文由桑尧热喷涂网收集整理。本站文章未经允许不得转载；如欲转载请注明出处，北京桑尧科技开发有限公司网址：http://www.sunspraying.com/
|
